The Diurnal Pattern of Nitrate Uptake and Reduction by Spinach (Spinacia oleracea L.)

Spinach plants were grown in bowls of aerated nutrient solutionin a controlled environment chamber for 24 h, and harvestedevery 3·5-5 h to record their growth, nitrate and wateruptake, and plant nitrate concentration. Twelve such experimentsare described, either with a 14/10 h dark/light regime, or continuouslight or darkness. The irradiance was either 110, 320, or 510µmol m^-2 s^-1 (PPFD). All these regimes began at the endof the light period of a 14/10 h dark/light regime (510 µmolm^-2 s^-1) lasting approximately 2 weeks. Nitrate uptake rate per g of dry weight of plant continued almostunabated at about 17 µmol h^-1 through the initial 14-hdark period, and then fell away sharply if the light was notrestored, but increased slightly when it was. With continuouslight at 510 µmol m^-2 s^-1, uptake rate rose steadily forthe first 24 h of light, and then fell sharply for about 6 h.Shoot nitrate concentration increased about three-fold in thedark phase, and declined in the light at a rate which was positivelyrelated to the irradiance. Root nitrate concentration was severaltimes higher than that of the shoot: its diurnal change wassmaller (relative to the mean) than that of the shoot. Nitratereduction occurred to a small extent in the dark, and increasedrapidly as soon as the lights came on, to remain at a roughlyconstant rate (related to the irradiance) throughout the lightphase. Dry matter increase in the light was related to irradiance,but with little increase above 320 µmol m^-2 s^-1. Respiratoryweight loss in the dark was not detectable. Rate of fresh weightincrease was approximately constant throughout light and darkperiods. The results compare quite well with the predictions of a simplesimulation model, based on the pump/leak principle.Copyright1994, 1999 Academic Press Spinacia oleracea, nitrate, uptake, reduction, influx, efflux, diurnal, regulation, model, simulation     

NITRATE REDUCTASE IN PHOTOSYNTHETIC BACTERIUM, RHODOSPIRILLUM RUBRUM. ADAPTIVE FORMATION OF NITRATE REDUCTASE

1. A method was discovered for adapting the cells of Rhodospirillumrubrum to grow on a nitrate medium, a capacity initially lackingin the organism. The adapted cells were able to grow with nitrateas the sole source of nitrogen. The growth responses of theadapted cells towards various nitrogenous sources were investigatedunder various conditions of incubation (aero- and anaerobiosis,light and dark).
2. The adapted cells were found to have simultaneouslyacquiredthe capacity for reducing nitrite and hydroxylamineas wellas nitrate. The path of nitrogen in the adapted cellswas assumedto be as follows: NO₃^– NO₂^– NH₂OH CellularNitrogen.
3. Nitrate metabolism of the adapted cells was investigatedundervarious conditions. In the light, nitrate was reducedand furtherassimilated, leaving insignificant amounts of nitritein themedium. In this case, consumption of nitrate was markedlyinhibitedby other forms of nitrogen (e.g., nitrite, hydroxylamine,aminoacids and ammonium salts). In the dark, nitrate was reducedas the terminal hydrogen acceptor in the oxidative breakdownof organic substances (e.g., malate) in the medium (i.e., nitraterespiration). More nitrite was accumulated in this case thanin the light. Molecular oxygen inhibited the reduction of, aswell as the growth on, nitrate in any of the above cases.
4. Theeffects on the rate of nitrate reduction (and respiratoryoxygenuptake) caused by various experimental factors (pH, nitrateconcentration, electron donors, and addition of hydroxylamine)were investigated, using the resting cells of the adapted organism.

¹ This paper was submitted to the University of Tokyo to fulfillthe requirement for the author's doctorate. ² Present Address: Botanical Institute, Kyoto University, Sakyo-ku,Kyoto. (Received February 14, 1963; )     

Diurnal regulation of NO3- uptake in soybean plants I. Changes in NO3- influx, efflux, and N utilization in the plant during the day/night cycle

The effect of light on NO₃^– utilization was investigatedin non-nodulated soybean (Clycine max L. Merr., cv. Kingsoy)plants during a 14/10 h light/dark period at a constant temperatureof 26C. A 30–50% decrease of net NO₃^– uptake ratewas observed 2–6 h after the lights were turned off. Thiswas specifically due to an inhibition of NO₃^– influx asmeasured by ¹⁵N incorporation during 5 min. The absolute valuesof NO₃^– efflux depended on whether the labelling protocolinvolved manipulation of the plants or not, but were not affectedby illumination of the shoots. Darkness had an even more markedeffect in lowering the reduction of ¹⁵NO₃^– in both rootsand shoots, as well as xylem transport of ¹⁵NO₃^– and reduced¹⁵N. Concurrently with this slowing down of transport and metabolicprocesses, accumulations of NO₃^– and Asn were significantlystimulated in roots during the dark period. These data are discussedin view of the hypothesis that darkness adversely affects NO₃^–uptake through specific feedback control, in response to alterationsin the later steps of N utilization which are more directlydependent on light. Key words: Glycine max, light/dark cycles, nitrate uptake, nitrate reduction     

The Effect of Inhibitors of Photosynthetic and Respiratory Electron Transport on Nitrate Reduction and Nitrite Accumulation in Excised Z. mays L. Leaves

The assimilation of nitrate and nitrite under dark and lightconditions in Zea mays L. leaves was investigated. Nitrate wasassimilated under dark-aerobic conditions. Anaerobiosis stimulatednitrate reduction and nitrite accumulation under dark conditions.Vacuum infiltration of inhibitors of respiratory electron transport,antimycin A and rotenone, stimulated nitrate reduction and nitriteaccumulation under dark-aerobic conditions. Vacuum infiltrationof low concentrations of PCP, DNP and mCCCP depressed nitratereduction and nitrite accumulation under dark-aerobic conditions,whereas, infiltration of higher concentrations stimulated nitratereduction and nitrite accumulation. The greatest level of nitrateand nitrite reduction occurred under light conditions. The inhibitorof photosynthetic electron transport, DCMU, stimulated the accumulationof nitrite in the light, but decreased nitrate reduction. Whenthe inhibitors of respiratory electron transport antimycin Aand rotenone, were supplied together with DCMU in the light,nitrite accumulation was enhanced. Low concentrations of mCCCPdecreased both nitrate reduction and nitrite accumulation underlight conditions when supplied with DCMU. Key words: Nitrate reduction, Nitrite accumulation, Leaves     

Short-Term Nitrate Effects on Hydroponically-Grown Soybean cv. Bragg and its Supernodulating Mutant: I. CARBON, NITROGEN AND MINERAL ELEMENT DISTRIBUTION, RESPIRATION AND THE EFFECT OF NITRATE ON NITROGENASE ACTIVITY

A comparison between two hydroponically-grown soybean genotypes(Glycine max [L.] Merr.) cv. Bragg and the supernodulating mutantnts 1007 was made in terms of dry matter accumulation, carbon,nitrogen, and mineral element distribution, ¹⁵N natural abundanceand the effect of short-term treatment with 4·0 mol m^–3KNO₃ on nitrogenase activity and respiration. Differences weremost pronounced in nodule dry weight and plant nitrogen content,both of which were recorded to be substantially elevated inthe mutant. Mineral element concentrations in different plantparts proved to be rather similar with the exception of Ca,found to be lower in leaves of the mutant, and Mn concentrationswhich were twice as high in roots of nts 1007. The values of¹⁵N natural abundance showed that both genotypes were equallydependent on nitrogen fixation when nitrate was absent. Theresults of the acetylene reduction assays indicated similarspecific nodule activity, while on a per plant basis nitrogenaseactivity of the mutant proved to be more than twice the amountof Bragg. This effect was also reflected in higher nodule respirationwhile root respiration remained below that of Bragg. Nitrate induced a substantial reduction in nitrogenase activitynot only in Bragg, but also in nts 1007. Nodule respiratoryactivity of Bragg was reduced by nitrate from 1·27 to0·34 mg C h^–1 plant^–1. In nts 1007 correspondingvalues were 2·70 to 1·52 mg C h^–1 plant^–1.Starch concentration in nodules was decreased in both genotypes,but nevertheless remained higher in nts 1007. Values for solublesugars in nodules even increased in the mutant in response tonitrate while the same treatment caused a reduction in Bragg.The data indicate that nitrogenase activities of Bragg and nts1007 are equally sensitive to short-term application of nitrate. Key words: Glycine max, C and N distribution, nitrate, root respiration, ¹⁵N natural abundance     

Variations in the Natural Abundance of 15N in Nitrogenous Fractions of Komatsuna Plants Supplied with Nitrate

Komatsuna (Brassica campestris L. var. rapa) plants were grownhydroponically under various conditions with respect to thesupply of nitrate, and the variations in levels of natural ¹⁵Nabundance (¹⁵N) in nitrogenous fractions of leaf blades, petiolesplus midribs, and roots in these plants were analyzed. The fractionation of nitrogen isotopes during uptake of nitratewas null irrespective of the concentrations of nitrate in theculture medium. The roots had lower ¹⁵N values than that inthe nitrate applied to plants. The nitrate in the three tissuesexamined had higher ¹⁵N values than that in the nitrate applied:the values were highest in the leaf blades which were presumedto have highest activities in terms of reduction of nitrate.In contrast, the amino acids and residual fractions had lower¹⁵N values than those in the nitrate applied. These resultssuggest that reduction of nitrate is a critical step in thefractionation of nitrogen isotopes in plant tissues in vivo. ¹Permanent address: Fukuoka Agricultural Experiment Station,Yoshiki 587, Chikushino, 818 Japan. (Received March 10, 1989; Accepted July 10, 1989)     

Carbon and Nitrogen Assimilation by Vicia faba L. at Low Temperature: the Importance of Concentration and Form of Applied-N

Low temperature (6 C) growth was examined in two cultivarsof Vicia faba L. supplied with 4 and 20 mol m^–3 N as nitrateor urea. Both cultivars showed similar growth responses to increasedapplied-N concentration regardless of N-form. Total leaf areaincreased, as did root, stem and leaf dry weight, total carboncontent and total nitrogen content. In contrast to findingsat higher growth temperatures, 20 mol m^–3 urea-N gavesubstantially greater growth (all parameters measured) than20 mol m^–3 nitrate-N. The increased carbon content per plant associated with increasedapplied nitrate or urea concentration, or with urea in comparisonto nitrate, was due to a greater leaf area per plant for CO₂uptake and not an increased CO₂, uptake per unit area, carbon,chlorophyll or dry weight, all of which either remained constantor decreased. Nitrate reductase activity was substantial inplants given nitrate but negligible in plants given urea. Neitherfree nitrate nor free urea contributed greatly to nitrogen levelsin plant tissues. It is concluded that there is no evidence for a restrictionin nitrate reduction at 6 C, and it is likely that urea givesgreater growth than nitrate because of greater rates of uptake. Vicia faba, broad bean, low temperature growth, carbon assimilation, nitrogen assimilation     

Transient nitrate uptake and assimilation in Skeletonema costatum cultures subject to nitrate starvation under low irradiance

Batch cultures of the nitrate-grown marine diatom, Skeletonemacostatum, were grown in various levels of nitrate supply, fromfull sufficiency of nitrate to 96 h of nitrate starvation Allthe cultures were maintained at low light intensity (50 µEm^–2 s^–1) to simulate the light regime of naturalphytoplankton populations in turbid waters or at the edge ofthe photic zone. The response of S costatum cells to perturbationswith 10 µM nitrate after variable starvation periods wasstudied At the deficiency-starvation borderline, the specieswas able to both increase its uptake rate and store intracellularnitrate pools. Surge, or initially high, uptake is characteristicof this situation. After long starvation periods the cells neededto acclimate to a nitrate environment before being able to utilizeit The time required for this was proportional to the previousstarvation period. Time courses of nitrate uptake and reductionwere strongly non-linear. Nitrate excretion was high (up to3 µM) under intermediate (24 and 36 h) starvation periods.Differences in the rates and times of uptake, reduction andassimilation produced strong uncoupling of the three processesThe results suggest that there is an adaptation to nitrate utilizationunder low light, nitrate shortage and discontinuous, suddennitrate inputs.     

The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

NITRATE REDUCTASE IN PHOTOSYNTHETIC BACTERIUM, RHODOSPIRILLUM RUBRUM. PURIFICATION AND PROPERTIES OF NITRATE REDUCTASE IN NITRATE-ADAPTED CELLS

1. From nitrate-adapted cells of Rhodospirillum rubrum, an activepreparation of nitrate reducing enzyme was isolated in partiallypurified state. The enzyme was found to be localized in thechromatophores of the cell and, on sonication, readily releasedinto the upernatant fraction. The purified enzyme, catalyzingthe electron transfer between DPNH and nitrate, contained ab-type cytochrome, flavin and non-heme iron, which was removedon dialysis in the presence of cyanide. Besides DPNH, only methylviologen(reduced form) was effective as electron donor. 2. The effects of pH and the addition of various activatorsand inhibitors on the rate of nitrate reduction were investigated,using DPNH or reduced methylviologen as the electron donor.The oxidation-reduction of the flavin and the heme in the enzymewas followed spectrophotometrically. A pathway of electron inthe nitrate reduction through this enzyme was proposed. 3. The nitrate reductase of this bacterium was compared withother nitrate reductases obtained from other sources, and themetabolic roles of this enzyme were discussed. In the nitrate-adaptedcells of Rsp. rubrum, only one and the same enzyme was obtainedunder different growth conditions of nitrate assimilation (i.e., nitrate as N-source; light as energy source) and nitrate-respiration(i. e., in the dark; nitrate as hydrogen acceptor and N-source). ¹ Dedicated to Prof. H. TAMIYA on the occasion of his 60th birthday.This paper was submitted to the University of Tokyo to fulfillthe requirement for the author's doctorate. ² Present address; Botanical Institute, Kyoto University. (Received December 14, 1962; )     

The uptake and flow of C, N and ions between roots and shoots in Ricinus communis L. IV. Flow and metabolism of inorganic nitrogen and malate depending on nitrogen nutrition and salt treatment

Ricinus plants were supplied with nutrient solutions containingdifferent N-sources or different nitrate concentrations andwere also exposed to mild salinity. Between 41 and 51 d aftersowing, the ratio of inorganic to total nitrogen in xylem andphloem saps, the content of inorganic nitrogen and malate intissues, and nitrate reductase activities were determined. Theflows of nitrate, ammonium, and malate between root and shootwere modelled to identify the site(s) of inorganic nitrogenassimilation and to show the possible role of malate in a pH-statmechanism. Only in the xylem of nitrate-fed plants did inorganicnitrogen, in the form of nitrate, play a role as the transportsolute. The nitrate percentage of total nitrogen in the xylemsap generally increased in parallel with the external nitrateconcentration. The contribution of the shoot to nitrate reductionincreased with higher nitrate supply. Under salt treatment relativelymore nitrate was reduced in the root as compared with non-treatedplants. Ammonium was almost totally assimilated in the root,with only a minor recycling via the phloem. Nitrate reductaseactivities measured in vitro roughly matched, or were somewhatlower than, calculated rates of nitrate reduction. From therates of nitrate reduction (OH -production) and rates of malatesynthesis (2H⁺-production) it was calculated that malate accumulationcontributed 76, 45, or 39% to the pH-stat system during nitratereduction in plants fed with 0.2, 1.0 or 4.0 mM nitrate, malateflow in the phloem played no role. In tissues of ammonium-fedplants no malate accumulation was found and malate flows inxylem and phloem were also relative low. Key words: Ammonium, Ricinus communis, phloem, xylem, transport, nitrate, nitrate reductase, nitrogen assimilation, malate     

Physiological Studies of the Sulpholipids of Diatoms

The sulpholipids of three species of freshwater and marine diatomNitzschia palae Kutz, Navicula muralis Lewin and Navicula incertaGrün, have been investigated under various culture conditions.The plant sulpholipid, sulphoquinovosyl diglyceride, was predominantlysynthesized in the light rather than in the dark while the unknownsulpholipids, designated as U₁ and U₂, were produced more inthe dark than in the light. It was found that cells starvedof carbon or sulphate utilized their sulpholipid reserve assources of these materials. Generally, cultures incubated inthe light and bubbled with air (with or without CO₂) showeda high level of incorporation of ³⁶S into sulpholipids. In culturesbubbled with oxygen-free nitrogen the incorporation of tracerwas very small. The photosynthetic and respiratory inhibitors,DCMU and DNP appreciably reduced the amount of tracer incorporatedinto the sulpholipids.     

Production of dissolved organic nitrogen during uptake of nitrate by Synedra planctonica: implications for estimates of new production in the oceans

Nitrate uptake was measured in batch cultures of the unicellularalga Synedra planctonica. During the light period, extensiveexcretion of dissolved organic nitrogen occurred, which representedup to 63% of nitrate uptake between 2 and 4 h into the lightperiod. During the following dark period, most of this excretednitrogen was taken up by dividing cells. New production canbe underestimated by 50% if such a phenomenon occurs duringuptake measurements restricted to the light period.     

Transport of Nitrate and Calcium into Legume Root Nodules

Nitrate transport into nodulated plants of soybean (Glycinemax), cowpea (Vigna unguiculata) and faba bean (Vicia faba)was investigated. Nitrate entering the root system of soybeandid not pass out of the vascular system into nodular tissuesin detectable quantities. On the other hand, nitrate could passfrom soil through the outer surface of nodules but did not penetratethe infected tissue. Similarly, nitrate was restricted to corticaltissues of cowpea and faba bean. Thus, nitrate cannot inhibitnitrogen fixation as a result of reduction to nitrite by nitratereductase within the bacteroid zone. These results are, however,consistent with an effect of nitrate on an oxygen diffusionresistance located in the nodule cortex. Unlike nitrate, measurable quantities of ⁴⁵calcium were transportedvia the xylem into infected and cortical tissues of soybeannodules: it also passed from the soil into the free space ofthe nodule cortex. Key words: Nitrate, legume nodules, calcium     

Light-Dependent CO2 Retrieval in Immature Barley Caryopses

Immature detached caryopses from barley (Hordeum vulgare L.var. distichum cv. Midas) were shown to be capable of light-dependentretrieval of internally-produced CO₂. In the first set of experiments,caryopses were radioactively labelled by supplying (U-14C)-sucroseto detached ears in liquid culture. Caryopses were then removedfrom the ear and given a 12 h chase of non-radioactive sucrosein either the light or dark. More ¹⁴C was recovered in the caryopsesafter the chase in the light than in the dark but the differenceswere not significant. In the second set of experiments, ¹⁴C-labelledcaryopses obtained by a 15 min light incubation in ¹⁴CO₂ weremaintained in either the light or dark for 3 h and any redistributionof label between the tissues recorded. The results show thatunder these conditions, photosynthesis in the Chl-containinggreen layer of the pericarp can prevent losses of internally-producedCO₂, since 3 times as much radiocarbon remained in the caryopsesincubated in the light as in the dark. These differences weresignificant at P=0.001. Experiments with the mutant barley Albinolemma, which has no Chi in the pericarp, showed that there waslittle difference between light and dark treatments. This confirmsthe suggestion that photosynthesis in the pericarp of the normalcultivar Midas may be concerned in the refixation of CO₂. Key words: Barley, pericarp, photosynthesis, carbon dioxide     

Short-Term Nitrate Effects on Hydroponically-Grown Soybean cv. Bragg and its Supernodulating Mutant: II. DISTRIBUTION AND RESPIRATION OF RECENTLY-FIXED 13C-LABELLED PHOTOSYNTHATE

Fully symbiotic or nitrate treated (3 d, 4·0 mol m^–3)soybean (Glycine max [L.] Merr.) cv. Bragg and a nitrate tolerantsupernodulating soybean mutant nts 1007 were exposed to ¹³Cenriched CO₂ for a period of 10 h. During this period and forthe subsequent 24 h, continuous measurements of ¹³CO₂ and ¹²CO₂evolution of their root systems were undertaken. Three harvestsduring the experiment allowed determinations of the distributionof recently fixed carbon in different plant organs. These measurementsindicated higher dependence of N₂ fixation in nts 1007 on recentlyfixed carbon (RFC) by showing elevated RFC concentrations innodules as well as their augmented respiration. Root respirationof both genotypes was generally more reliant on stored carbon. Nitrate induced in all measured parameters a clear responsein the mutant analogous to the wild type, but quantitative differencesremained throughout. Nodule respiratory activity, the relativespecific activity (RSA), and the utilization of RFC were substantiallyreduced, but remained higher in nts 1007 than in Bragg, whilethe demand of roots for RFC increased in Bragg more than inthe supernodulator. The elevated carbon requirement of the nodulecomplement of the mutant and a high dependence on recently fixedcarbon could be attributed to higher nodule growth and maintenancecosts of the supernodulating genotype and were not associatedwith augmented nitrogen fixation activity. This less efficientutilization of carbon and the associated almost parasitic characterof the nodule complement of nts 1007 is considered to be thecause of reduced growth of the mutant. No evidence was foundfor a physiologically based nitrate tolerance in terms of nitrogenfixation. Key words: Glycine max, nitrate, N₂fixation, respiration, carbon partitioning, steady-state labelling     

Uptake of 13C and 15N (ammonium, nitrate and urea) by Microcystis in Lake Kasumigaura

The uptake rate of carbon and nitrogen (ammonium, nitrate andurea) by the Microcystis predominating among phytoplankton wasinvestigated in the summer of 1984 in Takahamaira Bay of LakeKasumigaura. The V_max values of Microcystis for nitrate (0.025–0.046h^–1) and ammonium (0.15–0.17 h^–1) were considerablyhigher than other natural phytoplankton. The ammonium, nitrateand urea uptake by Microcystis was light dependent and was notinhibited with nigh light intensity. The K₁ values were farlower than the I_k values. The carbon uptake was not influencedby nitrogen enrichment. Microcystis accelerated the uptake rateby changing V_max/K _s value when nitrogen versus carbon contentin cells declined. Nitrate was scarcely existent in TakahamairiBay during the summer, when Microcystis usually used ammoniumas the nitrogen source. However, the standing stock of ammoniumin the water was far lower than the daily ammonium uptake rates.Therefore, the ammonium in this water had to be supplied becauseof its rapid turn-over time (–0.7–2.6 h).     

A 14N Nuclear Magnetic Resonance Study of Inorganic Nitrogen M?tabolism in Barley, Maize and Pea Roots

^l4N nuclear magnetic resonance (NMR) spectroscopy was used todetect simple nitrogen compounds in root tissues from barley(Hordeum vulgare), maize (Zea mays) and pea (Pisum sativum)seedlings. The spectra included contributions from amino andamide groups but, as expected, these peaks were broad and poorlydefined in comparison with the resonances from nitrate and ammonium.A quantitative analysis of the spectra of nitrate-grown barleyroots showed that the nitrate resonance intensity accountedfor the total nitrate content of the tissue. Timecourses ofthe tissue nitrate intensity in response to changes in the externalnitrate composition followed single exponentials, and couldbe discussed in terms of nitrate exchange and possibly reduction.There was no evidence from these data, or from spin-latticerelaxation measurements, for the existence of more than onelarge pool of nitrate in the roots, and the results were consistentwith a model in which the bulk of the nitrate is vacuolar. Incontrast, timecourses of the ammonium resonance in ammonium-grownbarley roots were biphasic under certain external conditions,suggesting some form of compartmentation of ammonium in thetissue. Key words: Nitrate, Ammonium, NMR, Compartmentation     

Nitrogen Assimilation in Barley (Hordeum vulgare L. cv. Mazurka) in Response to Nitrate and Ammonium Nutrition

Barley plants (Hordeum vulgare L. cv. Mazurka) were grown inaerated solution cultures with 2 mM or 8 mM inorganic nitrogensupplied as nitrate alone, ammonium alone or 1:1 nitrate+ammonium.Activities of the principal inorganic nitrogen assimilatoryenzymes and nitrogen transport were measured. Activities ofnitrate and nitrite reductases, glutamine synthetase and glutamatesynthase were greater in leaves than in roots but glutamatedehydrogenase was most active in roots. Only nitrate and nitritereductases changed notably (4–10 times) in response tothe different nitrogen treatments. Nitrate reductase appearedto be rate-limiting for nitrate assimilation to glutamate inroots and also in leaves, where its total in vitro activitywas closely related to nitrate flux in the xylem sap and wasslightly in excess of that needed to reduce the transportednitrate. Xylem nitrate concentration was 13 times greater thanthat in the nutrient solution. Ammonium nitrogen was assimilatedalmost completely in the roots and the small amount releasedinto the xylem sap was similar for the nitrate and the ammoniumtreatments. The presence of ammonium in the nutrient decreasedboth export of nitrate to the xylem and its accumulation inleaves and roots. Nitrate was stored in stem bases and was releasedto the xylem and thence to the leaves during nitrogen starvation.In these experiments, ammonium was assimilated principally inthe roots and nitrate in the leaves. Any advantage of this divisionof function may depend partly on total conversion of inorganicnitrogen to amino acids when nitrate and ammonium are givenin optimal concentrations. Hordeum vulgare L., barley, nitrate, ammonium, nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, glutamate dehydrogenase, nitrogen transport